Luminance is a key concept in lighting and optics, and understanding how to convert between lux (lx) and candela (cd) is essential for professionals in fields like lighting design, photography, and physics. These units are used to measure light intensity, but they describe different aspects of light.

In this guide, we’ll explain the difference between lux (lx) and candela (cd), how to convert between them, and provide practical examples and formulas to make the process easy to understand.

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Lux (lx) vs. Candela (cd)

What is Lux?

Lux (lx) is a unit of illuminance, which measures the amount of light that falls on a surface. It quantifies how bright the surface will appear when illuminated by light. Lux takes into account both the intensity of the light and the area over which the light is spread.

• 1 lux = 1 lumen per square meter (lm/m²).
• It is commonly used to measure how much light is reaching a specific area, such as the brightness of a room or outdoor space.

What is Candela?

Candela (cd), on the other hand, is a unit of luminous intensity, which measures the amount of light emitted by a source in a particular direction. It represents how bright a light source is, rather than how bright an area is illuminated by the source.

• 1 candela = 1 lumen per steradian (lm/sr).
• It is used to measure the intensity of light emitted by a light source, such as a light bulb, flashlight, or the Sun.

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Conversion from Lux to Candela

Conversion Relationship

To convert from lux (lx) to candela (cd), you need to understand the relationship between illuminance (lux), luminous intensity (candela), and distance.

The general formula for converting lux to candela is:

Candela (cd) = Lux (lx) × Distance² (m²)

Where:

• Lux (lx) is the illuminance measured at the surface.
• Distance (m) is the distance from the light source to the surface (in meters).

Why is Distance Important?

The distance between the light source and the surface matters because illuminance (lux) decreases with the square of the distance from the light source. Therefore, the further away the light source is from the illuminated surface, the lower the lux value will be, even though the candela value remains constant.

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Step-by-Step Instructions for Conversion

Step 1: Measure the Illuminance (Lux)

Start by measuring the illuminance in lux at the surface you are interested in. This can be done using a lux meter or by knowing the luminous flux of the light source and the area it illuminates.

Step 2: Measure the Distance

Measure the distance from the light source to the surface where the illuminance was measured. This distance is usually in meters.

Step 3: Use the Formula

Apply the formula to convert lux to candela:

Candela (cd) = Lux (lx) × Distance² (m²)

Step 4: Perform the Calculation

Simply multiply the lux value by the square of the distance to obtain the candela value.

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Example Conversions

Example 1:

You have an illuminance of 100 lux measured on a surface located 5 meters from a light source. To find the luminous intensity in candela:

1. Input: Lux = 100 lx, Distance = 5 m
2. Formula: Candela (cd) = 100 lx × (5 m)²
3. Calculation: Candela (cd) = 100 × 25 = 2,500 candela

Result: The luminous intensity of the light source is 2,500 candela.

Example 2:

You measure 200 lux on a surface that is 3 meters from the light source. To find the luminous intensity:

1. Input: Lux = 200 lx, Distance = 3 m
2. Formula: Candela (cd) = 200 lx × (3 m)²
3. Calculation: Candela (cd) = 200 × 9 = 1,800 candela

Result: The luminous intensity of the light source is 1,800 candela.

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Lux to Candela Conversion Table

For quick reference, here’s a table to convert lux (lx) to candela (cd) based on different distances.

Illuminance (Lux)	Distance (m)	Luminous Intensity (Candela)
10	1	10
100	1	100
100	2	400
200	3	1,800
500	5	12,500
1,000	10	100,000
2,000	20	800,000

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Frequently Asked Questions (FAQs)

1. What is the difference between lux and candela?

• Lux (lx) measures the amount of light that falls on a surface (illuminance), while candela (cd) measures the light emitted by a source in a particular direction (luminous intensity). Lux depends on both the light source’s intensity and the distance from the surface, while candela measures the intrinsic intensity of the light source itself.

2. Can I convert directly between lux and candela?

• No, you cannot convert lux directly to candela without knowing the distance between the light source and the illuminated surface. The formula involves distance squared, as illuminance decreases with the square of the distance.

3. Why does distance affect the conversion from lux to candela?

• Distance affects the conversion because lux measures the amount of light reaching a surface, which decreases as the distance from the light source increases. However, candela represents the intrinsic brightness of the light source, which remains constant regardless of distance.

4. How can I measure lux and candela?

• Lux can be measured using a lux meter, which directly reads the amount of light hitting a surface. Candela is typically measured by using a photometer or goniophotometer to measure the luminous intensity of a light source.

5. What are the typical units for illuminance and luminous intensity?

• Illuminance is measured in lux (lx), while luminous intensity is measured in candela (cd).